1. Field of the Invention
This invention relates to apparatus for treating fine granular material such as raw feed material for producing Portland cement. In particular the invention relates to a reinforced suspension preheater arrangement having a flue connection from a kiln to the preheater with means for controlling the flow of kiln gases to the mixing chamber so that 0-100 percent of kiln gases can be bypassed around the preheater and prevent short circuiting of the raw meal to the kiln.
2. Description of the Prior Art
Known apparatus of the prior art that involves preheating finely divided raw material suspended in and moving generally counter to the flow of heated kiln exit gases flowing through one or more cyclone dust separators are disclosed in Czechoslovakian Pat. No. 48,169 of 1934; published German Patent application No. K 156,877 of 1940; U.S. Pat. Nos. 2,648,532 and 2,663,560 of 1953 and many others as identified in U.S. Pat. No. 3,441,258 of 1969.
Apparatus of the aforementioned type but which additionally provide one or more auxiliary burners to additionally heat the feed material after the feed material has been preheated but prior to the feed material entering into the kiln is disclosed in such U.S. Pat. Nos. as: 3,235,239; 3,452,968; 3,507,482; 3,752,455; 3,834,860; 3,843,314; 3,869,248; 3,873,331; 3,881,862; 3,891,382; 3,891,383; 3,904,353; 3,910,754; 3,914,098; 3,925,091; 3,926,651; 3,932,116; 3,932,117 and 3,975,148.
It is known from the prior art and disclosed in U.S. Pat. Nos. 3,869,248; 3,881,862; 3,904,353; 3,910,754 and 3,975,148 that a throttle resistance may be provided within the flue connected to the outlet of the kiln to make use of the suction force of a blower from a first stage cyclone separator to draw hot air from the cooler into the preheater furnace. However, the installation of the fixed throttle resistance has the disadvantage of increasing both the air flow resistance and the power consumption of the suction blower.
In preheater systems, a bypass system is sometimes provided whereby a portion or perhaps all of the kiln gas is extracted from the feed end of the kiln bypassing the preheater. However, in providing a fixed throttle resistance it is necessary to size the throttle such that when there is sufficient vacuum in the preheater, to achieve desired cooler flow, there is not too much kiln flow. The smaller that the throttle resistance is the more assurance there is that it will be possible to achieve the desired cooler flow. If the throttle or orifice is sized smaller than necessary, the kiln and cooler flows are then balanced by partly closing the cooler duct damper. This has the effect of increasing the overall system pressure drop because the throttle or orifice is too small. This increases the power required to operate the preheater fan. The selection of the correct fixed throttle or orifice size is based on estimates such as duct losses etc., thus fixed orifice sizing is based on estimates. In addition, a reinforced suspension preheater system with a bypass and having a requirement to operate at various amounts of bypass, immediately introduces the problem of varying flow through the throttle orifice. This complicates the design of a system to achieve an optimum throttle size since there is more than one design condition. Also, in a reinforced suspension preheater system, the velocity of the gas entering the bottom of the mixing chamber must be sufficient to pick up the material that falls out of the calciner and carry it up to the fourth stage cyclone. If the velocity of the gas entering the bottom of the mixing chamber is not sufficient, the material will fall through the throttle or orifice and enter the kiln thereby bypassing or short circuiting the normal path through the fourth stage cyclone. To maintain sufficient velocity through the throttle during operation at reduced capacity, or during bypass operation, a smaller throttle size may be necessary.